Are ready-to-eat salads ready to eat? An outbreak of Salmonella Coeln linked to imported, mixed, pre-washed and bagged salad, Norway, November 2013

We investigated a nationwide outbreak of Salmonella Coeln in Norway, including 26 cases identified between 20 October 2013 and 4 January 2014. We performed a matched case-control study, environmental investigation and detailed traceback of food purchases to identify the source of the outbreak. In the case-control study, cases were found to be more likely than controls to have consumed a ready-to-eat salad mix (matched odds ratio 20, 95% confidence interval 2.7-8). By traceback of purchases one brand of ready-to-eat salad was indicated, but all environmental samples were negative for Salmonella. This outbreak underlines that pre-washed and bagged salads carry a risk of infection despite thorough cleaning procedures by the importer. To further reduce the risk of infection by consumption of ready-to-eat salads product quality should be ensured by importers. Outbreaks linked to salads reinforce the importance of implementation of appropriate food safety management systems, including good practices in lettuce production.